1. Field of the Invention
This invention relates to carrier signal recovery, and more particularly to an injection locked oscillator for use with a carrier recovery circuit.
2. Background of Related Art
Radio signals are utilized in many wireless applications for carrying an information signal from a transmitter to a receiver. Often the underlying information signal being transmitted is modulated for transmission at radio frequencies. Many modulation techniques exist for radio frequency (RF) communications, e.g., amplitude modulation (AM), frequency modulation (FM), and quadrature amplitude modulation (QAM). In most modulation techniques, a carrier signal is required to carry the information signal, whether it be modulated in amplitude, frequency, and/or phase.
For instance, in FM, the information signal is added to or frequency modulated (FM) with the carrier signal, and amplified for transmission to the receiver. The transmitted signal thus comprises the original information signal impressed upon a predetermined carrier frequency, which may be any value, e.g., 10, 100 or 200 megahertz (MHz).
A carrier recovery circuit in the receiver detects and demodulates the received modulated signal by subtracting the carrier frequency from the received modulated signal to result in the original information signal at the receiver. This is a well known technique which requires that the exact carrier frequency be known and recreated at the receiver.
Two basic techniques are utilized to generate the carrier frequency at the receiver: either the carrier signal is generated locally at an expected frequency, or it is sensed based on the incoming modulated signal. In theory either technique can result in an accurate recreation of the carrier frequency at the receiver. However, real world conditions result in variances caused between the actual carrier frequency as ultimately received by the receiver, and the receivers locally generated or recreated carrier frequency. For instance, electronics are affected by temperature variations. Thus, as the transmitter or receiver varies in temperature (e.g., as it is taken outside, or as it warms up when it is first turned on), the actual carrier frequency and that generated at the receiver will vary from one another, resulting in a less than accurate demodulation of the modulated signal at the receiver, and thus a less than accurate recovery of the information signal at the receiver.
In the first technique, a locally generated carrier frequency is typically provided based on a crystal oscillator having a frequency equal to the expected carrier frequency. FIG. 4 shows the details of such a conventional crystal oscillator.
In FIG. 4, a crystal oscillator typically requires power and ground, and provides a predetermined clock frequency output. Some conventional devices further include an enable signal input for halting operation of the crystal oscillator (and perhaps external clocked circuitry).
The enable signal is inverted by an inverter 508, to connect a power source to inverter 500. The basis for the crystal oscillator is a small signal gain amplifier 550 formed by resistors 510, 514 and 512, and inverter 500. The frequency output of the amplifier is controlled by the value of crystal 516. The value of capacitors 502 and 504 connected between opposite nodes of the crystal 516 and ground depends on the chosen frequency. Buffer 506 amplifies the small signal output from the amplifier 550 (e.g., 500 millivolt (mV) logic) to higher voltage logic (e.g., 3 V logic).
Unfortunately, the locally generated oscillator must be an extremely stable oscillator, and extremely accurate to the center frequency of the transmitted carrier frequency. Even so, a locally generated carrier signal is not conventionally capable of sensing drift in the actual received modulated signal.
Locally generated carrier signals require a priori knowledge of the carrier frequency, and must be as accurate and stable as possible. Unfortunately, locally generated carrier signals suffer from a susceptibility to phase errors which degrade the demodulation performance of the carrier recovery circuit. For instance, a delay between the sensing of the modulated signal and a recreation of the carrier frequency often causes a phase delay deteriorating frequency synchronization between the carrier frequency and the recreated carrier frequency in the receiver. Thus, the receiver must recreate the exact frequency and phase of the carrier signal to avoid degradation of the demodulated information signal.
The second technique for generating a carrier frequency at the receiver is shown in FIG. 5. FIG. 5 shows the use of a phase-locked loop (PLL) 400 to sense a frequency and phase of the carrier frequency in the incoming modulated signal. FIG. 5 shows a conventional carrier and clock recovery circuit including a PLL. The PLL 400 provides a phase and frequency corrected recovered carrier signal to the phase/frequency detector 102 for comparison with the actually received modulated signal (which due to real world conditions contains noise in the form of phase and frequency variations).
In FIG. 5, a phase/frequency detector 102 receives both the incoming modulated signal on line 420 and the output of the PLL 400 at point 422. The phase/frequency detector 102 compares the phase and frequency of the received modulated signal on line 420 with the phase and frequency generated by the PLL 400 to detect the actual phase and frequency of the carrier frequency as is it is received in the receiver. This accurately determined carrier frequency is subtracted from the received modulated signal to result in an output of the recovered information signal.
In more detail, the received modulated signal is input to the PLL 400 at line 420. A band pass filter 408 band pass filters the input modulated signal such that sideband information beyond that desired is eliminated. A phase detector 406, charge pump 404 and loop filter 402 provide a comparative phase for the received modulated signal and the locally sensed carrier frequency, and generate a DC signal for control of a voltage controlled oscillator (VCO) 430. The VCO 430 outputs a particular frequency based on the voltage level of its control input.
While having certain advantages, the inclusion of a PLL in a carrier recovery circuit adds complexity and cost to a receiver. This is particularly of concern in lower end applications such as low end cordless telephones or other 10-100 MHz low power, wireless applications. Moreover, the inclusion of a PLL slows the acquisition time necessary to acquire phase lock with changes in the modulated signal, and thus limits the frequency, modulation, and overall performance of the receiver.
There is thus the need to provide a simple carrier recovery circuit which is capable of accurately and quickly detecting and regenerating the carrier frequency, in particular without requiring a PLL.